Ironically, the Darwin Centre Phase Two project is more of a tale of creation than evolution. The Natural History Museum’s Phase 2 in London features one of the city’s most architecturally radical sights-a functioning Cocoon. The design fits with science themes by the building’s namesake-Charles Darwin. One of the oldest of London’s most treasured buildings, the £75 million ($114 million) extension is the largest since the structure located to its South Kensington site in 1881. It also showcases a new architectural icon that is unique in its technical and creative endeavors.
For the Cocoon, Danish architect C. F. Møller approached the design literally by creating a silk bubble with a series of ivory-colored threads with a subtle sheen that criss-cross on the surface. To start the process a 65 meter long (213 feet), eight-story high structure was built by sprayed concrete specialist Shotcrete. Expanded metal mesh was fitted to a skeletal steel rebar framework and a 250 millimeter (9.8 inches) thick layer of concrete spray applied to create a 3,500 square meter (37,670 square feet) raw concrete surface.
The Cocoon also performs a genuinely protective job. The concrete structure itself provides thermal mass to inhibit any fluctuations to the inside temperature of the unit. The outer surface was insulated with EPS as well to shield the concrete as much as possible from external temperature variations. With the outer layer exposed to the sun on one side through the glass façade, movement of the surface was a worry.
“From our point of view, we wanted a single point of responsibility for the insulation and polished plaster package,” says Alan Grant of HBG Construction. “Only specialist contractor Armourcoat was capable of taking this on.”
A GOOD YEARArmourcoat became involved as a specialist at the design stage a year in advance of construction commencing, says O’Brien. Members of the company attended various design meetings to interface with other specialists, such as Shotcrete Ltd. and Brogan Scaffolding.
“Alan Grant of BAM (formerly HBG) Construction employed our product knowledge to ascertain sequence of work, program constraints and design capabilities all at one time under the direction of the Architect CF Møller,” says O’Brien. “This was the first time anything like this had been created so most of the hard work was in the build-up/preparation and key discussion stages.”
As it emulates a natural object, the Cocoon is totally free-form and proved difficult to define mathematically. A specialist surveying company was employed to survey the actual shape of the Cocoon and enable accurate comparisons with the 3-D model design. The next step was to extract all the data from the design model to establish the position of all the control joint lines and to then plot these onto the actual surface of the Cocoon. The control joints, or reveals, were aesthetic, in giving the cocoon a spun web look, as well as functional to allow some expansion and contraction. With a total of 2.8 kilometers (more than a mile and half) of both straight and curved control joints, each and every line had to be plotted at 0.5 meter intervals onto the surface of the Cocoon.
Once all the intersecting lines had been chased into the surface each of the 340 panels could have special beads applied to the edges. Armourcoat designed, developed and prototyped a special bead/shadow gap design that could achieve the required design aesthetic onto a double curved surface.
Armourcoat’s AntiCrack substrate preparation system was then applied. AntiCrack is a cost effective polymer and fiber-modified gypsum skim-coat plaster and provides a substrate that will keep the top layer in fine condition for many years without fear of shrinkage or cracking. Once the AntiCrack had cured and dried out, two layers of the company’s Keycoat were applied prior to the application of the final finish. The final finished effect of a giant silk Cocoon was achieved using ivory colored Armuralia polished plaster, also provided by Armourcoat. Armuralia provided a silky smooth finish with subtle tonal variations.
The finished Cocoon project is a milestone achievement, a strong design and technical solution befitting the historic and social reference of the Natural History Museum. Our thesis is that Darwin would’ve been proud.
Sidebar: Team PlayersProject: Darwin Centre Phase Two-“The Cocoon,” Natural History Museum, London
Specialist Contractor: Armourcoat Ltd.
Client: Natural History Museum
Architect:C. F. Møller, Denmark
Main Contractor: HBG Construction